1. Field of the Invention
The present invention generally relates to transmitting apparatus and, more particularly, is directed to a radio transmitter apparatus.
2. Description of the Prior Art
In a mobile telephone system, for example, a cellular phone system, the base station controls the transmitter power output of the mobile station according to the transmission distance from the mobile station to the base station to increase telephone subscriber capacity of the phone system.
To enable the transmitter power output to be varied, the transmitter output stage of the mobile station is constructed as shown in, for example, FIG. 1.
In FIG. 1, reference numeral 1 designates the telephone transmitter, 2 the transmitting circuit, 20 the power amplifier fabricated as a module, 3 a strip line, 4 the up-channel bandpass filter, and 5 the transmitting and receiving antenna.
When making an outgoing call, an audio signal from the telephone transmitter 1 is supplied to the transmitting circuit 2, in which it is converted into an FM signal Su at the up channel frequency. This FM signal Su is amplified in power by the power amplifier 20 and supplied through the strip line 3 and the bandpass filter 4 to the antenna 5, from which it is transmitted to the base station.
In FIG. 1, reference numeral 6 designates the down channel bandpass filter, 7 the receiving circuit, and 8 the telephone receiver. Upon reception, an FM signal at the down channel frequency from the base station is received at the antenna 5, and the received FM signal is supplied through the filter 6 to the receiving circuit 7, in which the audio signal is derived. This audio signal is supplied to the telephone receiver 8.
Further, in FIG. 1, reference numeral 9 designates a microcomputer, and the transmitting circuit 2 and the receiving circuit 7, etc. are controlled by the microcomputer 9.
In the power amplifier 20, reference numerals 21 to 23 designate transistors, and the FM signal Su from the transmitting circuit 2 is sequentially supplied to the transistors 21 to 23 and then supplied to the antenna 5. The transistor 21 is operated in class AB mode and the transistors 22 and 23 are operated in class C mode.
Further, a power source voltage+Vcc from a power source terminal 31 is supplied to the circuits 2, 7 and 9 and to the transistors 22 and 23 as an operating voltage.
A voltage dividing circuit 32 is connected between the power source terminal 31 and the ground to derive a plurality of divided voltages V1 to Vn, and these voltages V1 to Vn are supplied to a switching circuit 33. The switching circuit 33 is controlled by the microcomputer 9 and the switching circuit 33 selects a voltage Vi from the voltages V1 to Vn, the voltage Vi being supplied to a voltage comparator 34.
The FM signal Su from the power amplifier 20 is supplied to a detector 35, in which it is converted into a DC voltage Vd of level equal to the level (amplitude) of the FM signal Su. This voltage Vd is supplied to the comparator 34. The output of the comparator 34 is supplied to the transistor 21 as its operating voltage.
Accordingly, the voltage is fed back by the comparator 34 to establish the equality of Vd=Vi, i.e., to make the level of the FM signal Su from the power amplifier 20 equal to the level of the voltage Vi. Thus, the level of the FM signal Su can be controlled by controlling the switching circuit 33 by the microcomputer 9, thereby controlling the transmitter power output (see Published Searched Application Gazette No. 63-501258).
When the mobile station is used while being carried, it is driven by a built-in battery housed therein.
When the mobile station is mounted in an automobile and used, the antenna 5 is changed to the antenna of the automobile and the mobile station is driven by the battery of the automobile.
The built-in battery and the automobile battery are different in voltage and the voltage of the automobile battery is generally higher than that of the built-in battery. Moreover, the transmitter output power of the mobile station is larger when the mobile station is mounted in the automobile than when the mobile station is carried.
In the above transmitting circuit 2, the load impedance from the amplifier 20 to the antenna side is constant, for example, 50 .OMEGA. regardless of the level of the transmitter output power. Further, the power amplifier 20 is generally designed so as to provide maximum efficiency at maximum transmitter output power.
Accordingly, when the mobile station is installed on the automobile, the efficiency of the transmitter is maximized, while when the mobile station is carried, the efficiency of the transmitter is lower.
However, the base station demands from the mobile station the output power necessary for telephone communication so that, when the mobile station is carried, the mobile station produces the output power demanded by the base station regardless of the lower efficiency of its transmitter. Thus, the service life of the built-in battery, and hence the available telephone communication time, is unavoidably reduced.